1. Field of the Invention
This invention pertains to parametric sound projection devices which incorporate acoustic heterodyning as basis for generating audio output. More particularly, the present invention relates to a device and method for enhancing a directional parametric speaker power output.
2. State of the Art
Recent developments have been made involving sound propagation from parametric speakers, acoustic heterodyning, and other forms of modulation of multiple ultrasonic frequencies to generate a new frequency. In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range or subsonic range. The resulting compression waves are projected within the air as a nonlinear medium. The present inventor has succeeded in advancing parametric audio devices from a state of curious research to commercially acceptable speaker systems which offer unique advantages over all other forms of audio devices. Applications are now expanding to general audio systems for home, office and automobile, military communications systems, weapons devices, point of purchase advertising sources and numerous other specialty devices.
A brief explanation of the theoretical parametric speaker array is provided in “Audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design” by Yoneyama et al as published in the Journal of Acoustic Society of America, 73(5), May 1983. Although technical components and the theory of sound generation from a difference signal between two interfering ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. This weakness in the prior art remained despite the assembly of a parametric speaker array consisting of as many as 547 piezoelectric transducers yielding a speaker diameter of 40–50 cm. Virtually all prior research in the field of parametric sound has been based on the use of tightly clustered conventional ultrasonic transducers, typically of bimorph character.
A common structural feature of prior art attempts to develop an effective parametric speaker is to form a substantially continuous array of transducers across the surface of a support plate. The natural assumption appears to be that filling in the interior area of the support plate with the maximum number of transducers is appropriate to maximize sound pressure level (SPL). Conventional speaker theory would suggest that increasing the number of transducers would indeed contribute to increased SPL. Accordingly, prior art parametric speakers are typically illustrated with bimorf transducers compactly positioned in honeycomb array. Until demonstration of the parametric ring concept as set forth in the parent patent applications, a general perception has existed that an increase in emitter surface was a primary factor for increasing SPL of a parametric emitter device.
Although not related to parametric audio production as presented in this application, a prior examination in the US Patent and Trademark Office of a parent application of the present invention discussed U.S. Pat. No. 4,418,248 by Mathis. The Mathis patent illustrates (patent FIGS. 1, 2 and 3) stereophones which are designed for operation in the audio spectrum. The inner radius 13 comprises a diaphragm which is designed to operate at audio frequencies greater than 1000 Hz. This frequency range corresponds to the upper audio frequency band. A second transducer comprising diaphragm 25 covers the lower frequency range of the audio spectrum. See column 3, lines 13 through 17. The outer radius 33 simply comprises ports which pass the lower frequency range audio vibrations within the stereo headphone. Accordingly, both the inner and outer radii are merely transmitting audio sounds in a conventional manner to enhance both upper and lower audio bandwidth.
In contrast, a parametric speaker projects ultrasonic emissions which are decoupled within the air for audio output. The principles of operation between conventional audio speakers as represented by Mathis and the present invention which involves a parametric speaker are very unrelated. It is important to distinguish between (i) conventional audio speakers that directly propagate audio sound by vibration of a diaphragm at a corresponding audio frequency range and (ii) parametric speakers which vibrate a diaphragm at ultrasonic frequencies of 25 Khz or greater and demodulate the ultrasonic output in air to indirectly produce audio output. It will be apparent to those skilled in the art that a ring of emitters producing audio output directly into the air such as Mathis will not correspond to a ring of ultrasonic emitters whose output is pumped into the air, which then operates to decouple an audio sideband frequency as audio output. In the former case, the audio speaker operates as a two-dimensional or point source of origination of the sound. In the later parametric embodiment, the audio sound source is similar to a three-dimensional column of air molecules which project out like a beam of light from the ultrasonic emitter. In essence, this column of air becomes the vibrating speaker element. Accordingly, prior art versions of audio speaker rings would not be relevant to the dynamics involved in a parametric ring emitter.